The objective of these studies is to describe the evolution of the simian virus 40 (SV40) variants which arise in rhesus monkey kidney cell cultures persistently infected with this virus and to evaluate those variants with respect to properties that might affect the course of this chronic infection. Major emphasis will be directed towards the identification and characterization of the SV40 variants which help to modulate the infection and thereby convert the potentially acute infection, resulting in cell death, into a chronic one instead. This persistently infected cell culture is a model of chronic infections which occur in the whole organism.